This invention relates to a driver circuit for coupling output electric signals from a logic circuit, to other circuits located on a chip with the logic circuit, the driver circuit having sufficient power to drive logic signals through an environment of electric conductors having significant wiring capacitance and, more particularly, to a push-pull driver constructed for minimization of average power dissipation so as to allow a more dense arrangement of circuits on a semiconductor chip without exceeding cooling capacity of the chip.
In the construction of computer systems and other systems employing large arrays of logic circuits, it has proven advantageous to construct the circuits in dense arrays upon a semiconductor chip. The dense arrays allows more functions to be located on a single chip so as to facilitate the design of computers and systems including data processing and control systems. Also, the dense arrangement of components decreases transit times of signals propagating among circuits of the chip for a faster operation of the system.
The electrical conductors interconnecting the various circuits on a chip present capacitance to the output terminals of logic circuits on the chip. To obtain a fast transmission of signals among the circuits, it is the practice to employ driver circuits at the output terminals of the logic circuits to boost the power output of the logic circuit sufficiently to charge the capacitors represented by the intercircuit wiring. By impressing large currents upon the intercircuit wiring, the driver circuits are able to overcome the capacitance effects of the intercircuit wiring and provide for rapid transmission of signals among the circuits.
By way of example in the form of circuits employed on a computer chip, a cascode current switch is a common form of logic circuit, and an emitter follower is a common form of driver circuit. Typically, the emitter follower circuit is teamed with the cascode circuit to apply output signals of the cascode circuit to other circuits on the chip. The emitter follower circuit includes, by way of example, a bipolar transistor and a resistor connected to the emitter terminal of the bipolar transistor. During a portion of a logic signal, the transistor is placed in a conduction mode to drive current through the resistor to produce a voltage drop across the resistor, the voltage drop representing the output signal which is applied to the other circuits. This is sometimes referred to as the pull-up voltage, and occurs relatively rapidly. During a further portion of the logic signal, the transistor is placed in a state of nonconduction to allow the current and the voltage drop across the resistor to drop to zero. This is sometimes referred to as the pull-down voltage. However, during the pull-down voltage stage of the output signal, the voltage drop occurs at a rate dependent on the wiring capacitance and the resistance of the resistor which serves to discharge the wiring capacitance. Thus, the pull-down voltage may occur relatively slowly unless the resistance is sufficiently small for a rapid discharge of capacitance.
A problem arises in the use of a small emitter resistor because the resistor draws much current with a resultant large amount of power dissipation. As a result of the large amount of power dissipation, the use of the emitter-follower driver circuits produces inefficiency in the operation of the circuit chip and increase the burden of cooling equipment in attempting to maintain the chip temperature at a safe operating temperature. In particular, it is noted that the large power drain of the emitter follower circuits limits the density of circuits to be constructed on a single chip because of excessive temperature rise during operation of the chip. One solution to this problem is the use of a push-pull driver disclosed in Dansky, U.S. Pat. No. 4,605,870, the teachings of which are included herein by reference. The present invention represents an improvement of the circuit disclosed in that patent.